1. Field of the Invention
This invention relates to an electric furnace, particularly an electric arc furnace, provided with a liquid cooling device for cooling thermally highly stressed wall parts of the furnace vessel, including essentially vertical cooling pipes which are series-connected in groups and through which liquid flows, wherein a bypass opening which short-circuits the cooling conduits at least partially is provided in the upper part of the vessel between adjacent cooling conduits.
2. Description of the Prior Art
Such a furnace is disclosed in Swiss patent application 3280/81-4 of 5-20-1981, which particularly teaches simple solutions for removing from the cooling system gas bubbles which are produced by local overheating and can adversely affect the cooling action or even result in the destruction of the cooling device, by positioning bypass openings in the upper part of the series-connected, vertical cooling pipes.
The publication of Korf-Fuchs Systemtechnik; "Stahlerzeugung Wasserkuhlsysteme fur Lichtbogenofen", translated, "Steel Production Water Cooling Systems for Arc Furnaces", undated, teaches cooling boxes which can be built in individually or as complete cooling systems for forming vessel walls in an arc furnace boiler. Constructive measures are intended to prevent cooling water from entering into the furnace area if the cooling boxes are perforated. A protective coating of fireproof material which is relatively thin compared to the thickness of a traditional, uncooled vessel wall of fireproof material of an arc furnace is applied to the cooling box walls which face the inside of the furnace. This protective coating protects the cooling boxes against heat radiation and prevents too much heat from being removed from the smelting area. The protective coating is additionally reinforced during smelting by spatters of slag which are cast against the walls by the action of the arc, where they remain clinging. Camlike projections attached to the walls of the cooling boxes reinforce the adhesion of the fireproof material and of the slag spatters.
A similar water cooling of the vessel walls of arc furnaces is known from the publication of the Lectromelt Corporation; "Water-Cooled Panels", April, 1980.
Fireproof material can be saved if cooling boxes are used to cool the vessel walls of arc furnaces, but there is also the danger, given the relatively thin protective coatings on the walls of the cooling boxes, that they can loosen at certain spots in an uncontrolled fashion, e.g. by mechanical action during the charging process, by the action of iron or slag spatters during the smelting process or by thermal tensions inside the coatings as a result of inhomogeneous heat radiation, unequal cooling action or when the vessel walls cool down. The heat transfer and thus the heat loss is particularly great at the exposed areas where the metal surface of the cooling box is directly irradiated by arcs.
Moreover, the non-protected areas receive a greater thermal stress than the other, protected cooling box wall and hot spots can develop by the smelting in two-shift or three-shift operation which is normally continuous in steel plants and foundries without being noticed by the operating personnel. In the most unfavorable instances, if they remain uncovered and the cooling conditions are unsatisfactory, these spots can become overheated to the extent that they result in perforations and severe associated consequences. Detection systems for monitoring cooling systems are complicated and expensive. If there were an indication of trouble, the furnace would then have to be taken out of operation so that the defective spots could be repaired.
In addition, the cooling walls of the cooling boxes which face the inside of the furnace, even though they are covered with a protective coating and were given a stress-free annealing before assembly, are constantly exposed to forces of expansion and contraction due to sharp variations of temperature. These forces particularly affect the corners and edges of the cooling surfaces, and thermal stresses arise in the welding seams connecting the cooling surfaces, in which tears can form under certain conditions which result in a breakthrough of water.